It is known how the continuous and diffuse use of fuels containing sulphurated substances, in the industrial, transport and home activities, has caused in the last decennia a considerable increase in the content of polluting substances in the atmosphere, among which is sulphur dioxide (SO.sub.2).
Such situation has resulted in a quick deterioration of a great many manufactured objects exposed to the atmosphere, through mechanisms of chemical attack already individualized, such as, for instance, sulphatization due to SO.sub.2.
Among the manufactured objects which have suffered and constantly suffer the attack of the atmospheric polluting agents, we can number the stone materials having prevailingly a carbonatic composition such as, for instance, marbles, sandstones, organogenic and sedimentary limestones.
The attacks usually show as considerable corrosions in the points subjected to rain washing away or in the form of patinas having a very variable color (black as well), which modify remarkably, in the non-washed away zones, the superficial look, the chromatic appearance, and the reflecting power of the stone materials.
The patina of the carbonatic material consists substantially of dihydrate calcium sulphate (CaSO.sub.4.2H.sub.2 O) coming from sulphur dioxide. As said dihydrate calcium sulphate is considerably more soluble than the other components of the stone materials, it is removed from the surfaces owing to rain washing away, leaving signs of evident corrosions.
Furthermore, dihydrate calcium sulphate shows a specific volume that is higher than that of carbonate, therefore the formation of dihydrate calcium sulphate becomes dangerous especially on sculptured parts and on materials having a heterogeneous surface and mass.
Moreover, there is the consideration that mortars having a prevailing or a whole content of lime which had been used diffusely until recently, both in plasters and in the binding of bricks and/or stones in masonry works, undergo the same phenomena of atmospheric decay as the carbonatic stone materials.
Consequently, we may note how masonry structures made of tiles and cement can be disfigured considerably because of patinas and scales of sulphatized material due to evaporation of flows of desultory rain washings away or of condensation of fogs and dews.
The same danger is represented by building elements consisting of carbonatic materials inserted in masonry structures made both of cement and of tiles.
Another considerable effect of sulphatization is the well-known effect due to the alteration of the pictorial layer of the frescos which, when coupled with the moisture action, can give rise to the complete ruin of the work.
The restoration of the damaged stone surfaces, both from an aesthetic and from a functional point of view, foresees the removal of the sulphatization patinas, and in the case of works having a remarkable artistic interest, it presents very delicate aspects.
A few of the most utilized techniques of restoration foresee abrasion actions, rinsings with water in large quantities coupled with brushings; furthermore, use is made of preparations the action of which is more or less known and controllable. Ammonium carbonate is, among the known reactants, one of the cheapest and most effective reactants for attacking calcium sulphate. Said ammonium carbonate is capable of reacting with calcium sulphate forming again calcium carbonate and ammonium sulphate through a double exchange reaction. Unfortunately, that does not lead to the restoration of the pristine surface of the stone material, as dihydrate calcium sulphate has a specific volume higher than that of carbonate and, therefore, the exchange reaction is generally coupled with a disgregation of the sulphatized layer.
In addition, the formed ammonium sulphate turns out to be disposable for a new sulphatization of other carbonatic zones of the stone materials and, therefore, at the end, the starting problem occurs again.
In the desulphatizing treatments of the frescos by means of ammonium carbonate, a second treatment, in spite of a few application problems, by means of barium hydroxide, was used successfully to stop the development of ammonium sulphate.